The present invention relates to an ultrasound diagnostic apparatus for scanning a three-dimensional region in an inside of a human subject (patient) with an ultrasound and creating a three-dimensional image in real-time on the basis of an echo signal obtained to allow it to be displayed.
The so-called three-dimensional scanning method for scanning a three-dimensional region in the inside of the human subject is typically divided into two methods. One method adopts a two-dimensional array type probe with a plurality of transducer elements arranged as a matrix array and moves an ultrasound beam vertically and horizontally across the three-dimensional region by an electronic operation, that is, a delay operation. The other method moves a one-dimensional array type probe manually or mechanically.
In order to scan the whole three-dimensional region for a short time period of, for example, 1/30 sec, while maintaining this in real-time, the former method is considered to become a mainstream in the future.
For the inspection of the circulatory system, in particular, the heart, such real-time three-dimensional imaging provides a three-dimensional configuration of the cardiac muscle, as well as information helpful to its motion and cardiac cavity volume, etc., to the observer. It is, therefore, expected that the diagnostic precision is remarkably improved. However, this method has few problems, that must be solved in order to obtain practical use.
Its major obstacle lies in that the ultrasound image is low in quality. In order to three-dimensionally display a region of interest of the heart, etc., with the use of a surface model and polygon model, it is necessary to have a process for extracting a contour of the region of interest. However, too low a quality of the ultrasound image causes many more extraction errors.
There are broadly two reasons for the low quality of ultrasound images obtained. One reason is that there occurs an artifact, that is, a false image, in the ultrasound image specific to, and not visible to, the X-ray computed tomography and magnetic resonance imaging. The generation of the artifact is due to the grating lobe, multiple reflection, lens effect in the patient, mirror effect, and so on. Another reason is that, due to the diffusion of the ultrasound, the temporal resolution of the ultrasound image is prominently degraded in comparison with the X-ray computed tomography and magnetic resonance imaging.
The two-dimensional array type probe indispensable to a three-dimensional real-time imaging involves much more transducer elements than the one-dimensional array type probe. If any channel is allocated individually to these transducer elements, the size of a transmitting/receiving circuit becomes vast, thus involving an increase in processing time. In an ordinary case, however, the ultrasound is transmitted and received using some of the transducer elements instead of using all the transducer elements. If their aperture width is narrowed, then the spatial resolution is lowered at a far distance and, therefore, the "partly cut" driving of the transducer elements is done by skipping these transducer elements in one or two interval units. Such driving promotes the generation of the grating lobe as will be set out below.
As well known, in order to cancel the grating lobe, the grating lobe is driven out of a visual range. This condition is given by: EQU EP&lt;.lambda./(1+.vertline.sin .theta..vertline.) (1)
where
.theta.: the maximal deflection angle of the main lobe; PA1 .lambda.: the internal (inside-patient) wavelength; and PA1 EP: the center-to-center distance between the transducer elements.
In general, the center-to-center distance EP of the transducer elements is so designed as to clear the condition. In the partly cut driving, however, the center-to-center distance EP of the transducer elements becomes effectively two- and three-times, so that the condition above cannot be cleared. As a result, the grating lobe enters into the visual field.